Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             reactive intermediates
               Class dipolar compounds
                 Class 1,3-dipolar compounds
                   Class Allyl type 1,3-dipolar compounds [X=Y+-Z <-> X-+Y=Z <-> X+YZ- <-> X-YZ+ (X,Z=C,N,O; Y=N,O)]
                     Class oxonium ylides [R2C=O+-Y]
                       Class carbonyl ylides [R2C=O+-CR2 <-> R2C+OC-R2]